A quorum is the minimum number of members of an organization who must be present in order for their meeting to be legal or official. The word is often used in legislative assemblies, corporations and societies who make official decisions. The by-laws of an organization will usually say how many members make up a quorum.
In the case of legislatures and government bodies, the requirements for a quorum are often set by statute or in their constitution. Some bodies use a fixed number for a quorum while others use a percentage of the members. It is usually the responsibility of the chairperson to make sure there is a quorum present. If there is not a quorum at a meeting, then in most cases the only business that can be discussed is taking steps to obtain a quorum, to decide on a time when to adjourn the meeting and to adjourn the meeting.
The noun quorum is the plural of the Latin word qui, meaning "of whom."
A similar term, "quorum-busting", is a tactic used by members of a group when they know they will lose a vote. If enough members do not show up for a meeting, a vote cannot take place if there is no quorum. It is a delaying tactic similar to a filibuster. Both are used in the hopes that if a vote is delayed long enough, it may not take place at all.